Lithium-ion batteries feature a high energy density, and rechargeable lithium-ion batteries are reusable and have been used in small apparatuses such as mobile phones and personal computers (PCs) for many years. Furthermore, the rechargeable lithium-ion batteries are possibly used as power supplies of electric vehicles. For a rechargeable lithium-ion battery currently available, except the cathode material and the anode material, the diaphragm and the solvent of the electrolyte solution are all formed of organics. However, the organics are reducing agents and are liable to react with the oxidizing cathode material in a recharging state to generate water and carbon dioxide, and this makes the cycle life of the battery too short to meet the requirement on the power supply of an electromobile. Moreover, the organics will react with oxygen in the air when the battery is destroyed (e.g., in case of an accident), and this may lead to such potential safety hazards as combustion or explosion. Additionally, because the cathode material can oxidize the electrolyte solution, the battery shall not be designed to have a too high voltage (which is generally lower than 4.2 V). Thus, the electric energy that can be stored by the battery in a unit volume is relatively low. By contrast, a solid battery has many advantages. Because the solid battery has no liquid electrolyte solution therein but has a solid electrolyte, the cathode material will not react with other substances even when the cathode material becomes a strong oxidant in the recharging state. Therefore, the solid battery, which features a long cycle life and a high volumetric energy density and a high gravimetric energy density, is safe to use and is capable of operating at a high voltage.
The solid battery is mainly comprised of a cathode material, an anode material and a solid electrolyte diaphragm, in which the solid electrolyte diaphragm is crucial. Because lithium ions have a slower transmission speed in the solid than in the electrolyte solution, it is crucial for the solid battery to research and develop a solid electrolyte material capable of rapidly transmitting lithium ions. The research result of Masahiro et al. (see Solid State Ionics 170: 173-180 (2004)) indicates that, LiSP, LiSiPS, LiGePS or some other compounds having a molecular formula LixM1-yM′yS4 (M=Si, Ge and M′═P, Al, Zn, Ga and Sb) has a lithium-ion-conduction capability similar to that of the electrolyte solution. Seino et al. disclose a solid electrolyte in US Application Publication 2009/0011339A1, which consists of Li2S, Li4SiO4, LiBO3 and Li3PO4 and has a good lithium-ion-conduction capability. Trevey et al. (see Electrochemistry Communication 2009, 11(2), 1830-1833) have reported a solid electrolyte prepared by Li2S and P2S5, which has a lithium-ion conductance of 2.2×10−3 Scm−1. In the processes of preparing these solid electrolytes, raw materials are all subjected to ball milling and high-temperature (750° C.) treatment, and are then milled into powders to produce batteries. The batteries need to be treated at a high temperature so that the solid electrolytes are densified. All the preparation processes of the solid electrolytes and the batteries must be carried out under anhydrous conditions. Such solid batteries have a complex manufacturing procedure, a long process flow and a too high manufacturing cost, so they are difficult to be commercialized.
Kugai (see U.S. Pat. No. 6,641,863) discloses a method for preparing a solid electrolyte diaphragm by using a laser to plate a film in the vacuum through vaporization. This method can be used to prepare a film of nano thickness, but has a particularly low speed when being used to prepare a film of micrometer-level thickness. Another shortcoming of the method is that, the method uses a very expensive target and needs to be carried out in the vacuum, which leads to a high manufacturing cost. Therefore, the method is unsuitable for producing lithium-ion batteries.
Oladeji discloses a method for preparing a solid electrolyte in US Applications 2011/0171398A1, 2011/0171528A1 and 2011/0168327A1. The methods comprise: dissolving a raw material in a solvent, and particularly dissolving acetate, sulphate, halide, citrate, nitrate and organic metal compounds of the raw material in water or an organic solvent to prepare a solution; then, spraying the solution onto a surface of a heated substrate to produce a film; under an electric field, spraying lithium ions onto the produced film for lithiation; and then, firing the resulting film at a temperature of 100° C.˜500° C. to obtain a solid electrolyte diaphragm. The methods disclosed in these patents are substantially to use aqueous solution to produce composite oxide solid electrolyte diaphragm, and it is difficult to use these methods to produce a lithium-ion metal composite sulfide film. However, in the solid electrolytes reported in the references and disclosed in patents or patent publications currently, the lithium-ion-conduction capability of lithium-ion metal composite sulfide solid electrolytes is higher by 1 to 2 orders of magnitude than that of lithium-ion metal composite oxide electrolytes.